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Nobody home? The search for extraterrestrial intelligence has a powerful new tool at its disposal, Australian scientists report.

For the first time, a group led by astronomer Professor Steven Tingay from Curtin University have used a sensitive type of radio telescope, known as a very long baseline interferometer, to listen out for radio signals coming from a distant planet.

For eight hours in June 2007, they trained one of these interferometers - the Australian Long Baseline Array - toward a nearby star known as Gliese 581, which is thought to have two potentially life-sustaining planets orbiting it.

Although the foray drew a blank, the researchers say their approach holds promise for the future. Their report, posted on the pre-press website arXiv.org, is due to be published in The Astronomical Journal.

"We've proved the concept," says Tingay. "Although it's a null result, it's good to be able to show that the technique works."

Very long baseline interferometers are well suited to the search for radio signals from space because they are made up of multiple telescopes separated by large distances. The three Australian Long Baseline Array dishes used in this instance were spread around NSW.

"One of the problems with the search for extraterrestrial intelligence is that if a telescope picks up a narrow band radio signal, ultimately you don't know if it's coming from a distant planet, or the TV antenna 20 kilometres away," explains Tingay.

The benefit of interferometers such as the ALBA - or the upcoming Square Kilometre Array - is that any human interference likely would be confined to one or two of those antennas, he says.

"Even if you did see it on more, you could very quickly eliminate it as coming from the part of the sky where the planet is."

Proof of concept

Tingay and colleagues, including PhD student Hayden Rampadarath, focused their search on a small part of the radio frequency spectrum - from 1230 to 1544 MHz.

"That's the region of spectrum that contains the hydrogen line emission, which people think is a reasonable place to look," says Tingay. "But it's a tiny, tiny sliver of the radio frequency."

The astronomers detected 222 potential alien signals during their observation. However they were able to rule out all of these using automated analysis techniques.

Still, the negative finding does not definitively show there are no alien civilisations orbiting Gliese 581, notes Tingay.

What it does mean is that during the period they were observing, there were no powerful radio signals directed from that planetary system toward Earth.

"We would have seen a signal brighter than 7 megawatts per hertz, but that is a pretty strong signal," he says. "Who knows, maybe it was a public holiday on that planet or something and we missed out."

The findings also show that the Square Kilometre Array would be a good addition to the hunt for alien intelligence, he notes.

"The real value of this is in showing the technique works and you could feasibly run this kind of program on the SKA for almost zero cost."